Recently, with the achievement of hard disk drives (HDD) with a high storage capacity, bearings of spindle motors which are used therefor have been demanded to enable the motors to rotate with high accuracy and with low power consumption, and employment of hydrodynamic bearings has been proposed. In a hydrodynamic bearing, the sliding surfaces of the bearing are in contact with each other when the motor is started and stopped, so that problems in terms of durability may occur depending on the material of the sliding surfaces, the machined condition thereof or the assembling accuracy. For this reason, the necessity to use a lubricant has been considered.
Generally, lubrication of the area of sliding contact between a shaft and a bearing metal of a hydrodynamic bearing used in a spindle motor for an HDD is effected by using a liquid lubricant, i.e., oil or grease (hereinafter referred to as "oil"), or various kinds of coating.
In a case where the bearing material is an ordinary metallic material, a lubricant film with a thickness in the range of from 0.01 micron to several microns is formed. However, the bonding between the metallic material and the lubricant is weak, so that the lubricant is likely to separate and splash to the outside in the form of an oil mist, thus making adverse effects on the magnetic medium of a hard disk or the like.
In a case where ceramic sliding surfaces that are finished with a surface roughness Ra of 0.2 microns, for example, are each coated with a film of a lubricant which is unlikely to splash to a thickness of 500 .ANG., for example, the thickness of the film thus formed is likely to be uneven, and it is difficult to control the lubricant film so that the film thickness will be uniform. If the film thickness exceeds 100 .ANG., a sticking phenomenon based on the meniscus effect of the lubricant occurs, so that the static frictional torque increases. In such a case, the spindle motor may be unable to start. On the other hand, if the lubricant film is excessively thin, the oil will separate during the repeated start and stop of the spindle motor, causing the ceramic sliding surfaces to come into contact with each other. In consequence, wear progresses, causing dust to be generated.
To solve these problems, the following methods have heretofore been proposed: According to one conventional method, the sliding surfaces are coated with a light and soft metal layer, e.g., silver, lead, white metal, etc., as a lubricating layer to prevent seizing by utilization of plastic deformation, shearing and sticking of the soft metal. With this method, dust that is generated from the soft metal as wear progresses forms a considerably large lump, and this dust may leak out or enter the area between the sliding surfaces or fill dynamic pressure generating grooves, thus making it impossible for the device to function as a bearing.
According to another conventional method, the sliding surfaces are coated with a material having self-lubricating properties, e.g., a polymer having lubricating properties, such as molybdenum disulfide, polytetrafluoroethylene, etc., by means, for example, of sputtering (ion plating or vapor deposition).
However, methods that employ a vacuum apparatus, such as a sputtering method, involve problems that the apparatus is complicated and setting of conditions is difficult and that it is impossible to effect uniform coating on the inner peripheral surface. Moreover, the polymer that is provided specially for lubricating purposes may lose its lubricating properties. In addition, as wear progresses, the lubricating layer may break suddenly, resulting in an increase in the coefficient of friction. In such a case, a large torque is needed at the time of starting.
Japanese Patent Public Disclosure No. 64-65322 discloses a hydrodynamic bearing in which at least one of surfaces which rotate face-to-face with each other is coated with either or both an organopolysiloxane having a functional group and a fluorine-containing polymer. In the hydrodynamic bearing disclosed in this publication, the above-described substance reacts with the surface of a metal that constitutes the sliding surface to form a lubricant film. However, since the increase in the degree of machining accuracy of a metal is limited and the surface roughness of a metal is relatively large, the lubricant film formed in the hydrodynamic bearing disclosed in the publication has a multilayer structure undesirably.